1. Technical Field
The present invention relates to a liquid crystal display, particularly to a liquid crystal display that has a light source, such as a backlight and a front light, whose brightness level can be automatically changed depending on the brightness of external light.
2. Related Art
The use of liquid crystal displays (LCDs) has been widespread not only for information and communications devices but also for general electric appliances. Among them, reflective LCDs that require no back or side lights (hereinafter collectively referred to as “backlights”), unlike transmissive LCDs, have been widely used for mobile applications in order to reduce power consumption. Since the reflective LCDs use external light as their light source, they present poor display visibility in a dark room. To address this problem, there have been developed reflective LCDs employing front lights (e.g., see JP-A-2002-131742) and semi-transmissive LCDs having a combination of characteristics of transmissive and reflective LCDs (e.g., see JP-A-2001-350158). For example, reflective LCDs employing front lights display images by turning on their front lights in dark places and by using external light without using the front lights in bright places. There is no need to keep the front lights on, whereby power consumption can be significantly low. Semi-transmissive LCDs have a transmissive part with a transmissive electrode and a reflective part with a reflective electrode in each of their pixels. They display images by using the transmissive part in each pixel with their backlights turned on in dark places and by using the reflective part bringing in external light without turning on the backlights in bright places. There is again no need to keep the backlights on, whereby power consumption can be significantly cut down.
Such reflective LCDs and semi-transmissive LCDs have variable visibility depending on the intensity of external light. To ensure a highly visible display, the end user needs to take the trouble to turn on or off the backlights or lower the light level after deciding whether to turn on the backlights according to the intensity of external light. Here, the end user may unnecessarily turn on the backlights even when sufficient external light is available. When this happens, waste power consumption increases to cause quick battery drain in cellular phones and other mobile appliances.
To solve this problem, some related art examples have been developed in which an ambient light photo sensor is provided to a LCD in order to detect the intensity of external light and control on/off of the backlights according to the detection results.
For example, the LCD described in JP-A-2002-131719 includes a photodetector unit having a thin-film transistor (TFT) as an ambient light photosensor on a LCD panel substrate. The TFT ambient light photo sensor is manufactured together with another TFT serving as a switching element for the panel. By detecting photo-leakage current from the TFT ambient light photosensor, the LCD automatically turns on/off its backlight depending on the brightness of the surroundings. A LCD described in JP-A-2003-215534 includes a photodiode as an ambient light photosensor, and a light-emitting diode as a backlight provided with temperature-compensated current depending on the brightness of the surroundings. JP-A-2004-007237 describes a light-emitting diode functioning as both a backlight and a device-operation indicator and an ambient light photosensor, whereby the backlight is controlled on/off based on the electromotive force of the diode depending on the brightness of the surroundings.
However, the LCD including a TFT ambient light photosensor integrally on its substrate as described in JP-A-2002-131719 may cause malfunction or reduced sensitivity under the influence of light emitted from the backlights. FIG. 12 is a sectional view schematically showing a TFT ambient light photosensor included in a known LCD. Referring to the drawing, the TFT ambient light photosensor includes a gate electrode GL, a source electrode SL, a drain electrode DL, and a semiconductor layer 19L made of amorphous silicon, for example. Provided on an active matrix substrate 2, the TFT ambient light photosensor detects light L1 incident from a display surface through a color filter substrate 25. The drawing also shows a capacitor C.
However, positioning of the TFT ambient light photosensor in this manner may generate light L2 incident on the semiconductor layer 19L, along with the light L1 incident from the display surface. This is because light emitted from a backlight BL is reflected on a layer 27, including a color filter and a black matrix, between a color filter substrate 25 and a common electrode 26. When the light L2 comes in, an extra dark current flows in the TFT ambient light photosensor, thereby causing malfunction or reduced sensitivity of the TFT ambient light photosensor.